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| United States Patent |
5,152,894
|
|
Haubs
, et al.
|
October 6, 1992
|
Semipermeable membrane made from a homogeneously miscible polymer blend
Abstract
A semipermeable membrane is described which comprises a homogeneously
miscible polymer blend of an aromatic polyamide and polyvinylpyrrolidone.
The aromatic polyamide is in particular formed of recurrent units of the
formula I:
##STR1##
wherein E.sup.1 and E.sup.2 are identical or different and are selected
from the groupings
##STR2##
--AR.sup.1 --, and --AR.sup.1 --X--AR.sup.2,
where AR.sup.1 and Ar.sup.2 are the same or different 1,2-phenylene,
1,3-phenylene or 1,4-phenylene groups which may be substituted by (C.sub.1
-C.sub.6)-alkyl, (C.sub.1 -C.sub.6)-alkoxy, --CF.sub.3 or halogen and X
denotes
a) a direct bond or one of the following divalent groups: --O--,
--C(CF.sub.3).sub.2 --, --SO.sub.2 --, --C(R.sup.1).sub.2 --, with R.sup.1
being hydrogen, (C.sub.1 -C.sub.6)-alkyl or fluoroalkyl having from 1 to 4
carbon atoms in the alkyl group or
--Z--AR.sup.1 --Z--, where Z is one of the groups --O-- and
--C(CH.sub.3).sub.2 or
--O--Ar.sup.1 --Y--Ar.sup.2 --O--, where Y has the meaning given under Xa)
above.
The membrane has pronouned hydrophilic properties and is moreover stable to
hydrolyzing agents and oxidants and to the action of heat. It is also
resistant to organic solvents and microorganisms and exhibits low
adsorption of proteins. A process for producing the membrane and a process
for modifying the retention capacity thereof are also described. The
membrane is produced by spreading a solution of the homogeneously miscible
polymer blend on a planar substrate and precipitating the blend. Retention
capacity is modified by heat treatment.
| Inventors:
|
Haubs; Michael (Bad Kreuznach, DE);
Kreuder; Willi (Mainz, DE);
Krieg; Claus-Peter (Frankfurt am Main, DE);
Wildhardt; Juergen (Huenstetten-Wallrabenstein, DE)
|
| Assignee:
|
Hoechst Aktiengesellschaft (Frankfurt am Main, DE)
|
| Appl. No.:
|
733377 |
| Filed:
|
July 22, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
210/500.38; 210/490; 210/500.42 |
| Intern'l Class: |
B01D 071/28; B01D 071/56 |
| Field of Search: |
210/490,500.38,500.42
|
References Cited
U.S. Patent Documents
| 3526588 | Sep., 1970 | Michaels et al. | 210/500.
|
| 3615024 | Oct., 1971 | Michaels | 210/490.
|
| 4051300 | Sep., 1977 | Klein et al. | 428/398.
|
| 4229291 | Oct., 1980 | Walch et al. | 210/23.
|
| 4720343 | Jan., 1988 | Walch et al. | 210/500.
|
| 4891135 | Jan., 1990 | Haub et al. | 210/500.
|
| 4900443 | Feb., 1990 | Wrasidlo | 210/500.
|
| 4906375 | Mar., 1990 | Heilmann | 210/500.
|
| 4935141 | Jun., 1990 | Buck et al. | 210/500.
|
| Foreign Patent Documents |
| 0077691 | Apr., 1983 | EP | 210/500.
|
| 0176992 | Apr., 1986 | EP | 210/500.
|
| 0219878 | Apr., 1987 | EP.
| |
| 0228072 | Jul., 1987 | EP.
| |
| 58-058106 | Apr., 1983 | JP | 210/500.
|
| 61-271006 | Dec., 1986 | JP | 210/500.
|
Other References
W. Pusch et al., "Synthetic Membranes: State of the Art", Desalination, 35
(1980) pp. 5-20.
Robert E. Kesting, "Synthetic Polymeric Membranes", 2nd Ed., 1985, pp.
237-286.
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Foley & Lardner
Parent Case Text
This application is a continuation of application Ser. No. 07/470,211,
filed Jan. 25, 1990, now abandoned.
Claims
What is claimed is:
1. A semipermeable membrane comprising a homogeneously miscible polymer
blend, wherein said homogeneously miscible polymer blend comprises
(i) a homo- or copolyaramide comprising at least one recurrent structural
unit of the formula I
##STR12##
wherein E.sup.1 and E.sup.2 are identical or different and are selected
from the groupings
##STR13##
--Ar.sup.1 --, and --Ar.sup.1 --X--Ar.sup.2,
where in E.sup.1 Ar.sup.1 and Ar.sup.2 are the same or different 1,4
-phenylene groups which may be substituted by (C.sub.1 -C.sub.6)-alkyl,
(C.sub.1 -C.sub.6)-alkoxy, --CF.sub.3 or halogen and X denotes
a) a direct bond or one of the following divalent groups: --O--,
--C(CF.sub.3).sub.2 --, --SO.sub.2 --, --CO--, --C(R.sup.1).sub.2 --, with
R.sup.1 being hydrogen, (C.sub.1 -C.sub.6)-alkyl or fluoroalkyl having
from 1 to 4 carbon atoms in the alkyl group, or
b) --Z--Ar.sup.1 --Z--, where Z is one of the groups --O-- and
--C(CH.sub.3).sub.2 --, or
c) --O--Ar.sup.1 --Y--Ar.sup.2 --O--, where Y has the meaning given under
Xa) above,
and in E.sup.2 Ar.sup.1 and Ar.sup.2 are the same or different
1,2-phenylene, 1,3-phenylene or 1,4-phenylene groups which may be
substituted by (C.sub.1 -C.sub.6)-alkyl, (C.sub.1 -C.sub.6)-alkoxy,
--CF.sub.3 -- or halogen and X denotes
d) a direct bond or one of the following divalent groups: --O--,
--C(CF.sub.3).sub.2 --, --SO.sub.2 --, --CO--, --C(R.sup.1).sub.2 --, with
R.sup.1 being hydrogen, (C.sub.1 -C.sub.6)-alkyl or fluoroalkyl having
from 1 to 4 carbon atoms in the alkyl group, or
e) --Z--Ar.sup.1 --Z--, where Z is one of the groups --O-- and
--C(CH.sub.3).sub.2 --, or
f) --O--Ar.sup.1 --Y--Ar.sup.2 --O--, where Y has the meaning given under
Xd) above, and
(iii) polyvinylpyrrolidone having a molecular weight, indicated as the
weight average, of about 50,000 to 100,000.
2. A membrane as claimed in claim 1, wherein halogen comprises fluorine,
chlorine or bromine.
3. A membrane as claimed in claim 1, wherein the grouping E.sup.1 comprises
identical or different structural units and denotes a 1,4-phenylene group
or the group
##STR14##
4. A membrane as claimed in claim 1, wherein the grouping E.sup.2 comprises
identical or different structural units and denotes the 1,4-phenylene
group or the group
##STR15##
wherein R.sup.2 denotes a lower alkyl or alkoxy group having up to 4
carbon atoms each in the alkyl group or F, Cl or Br or the group
##STR16##
in which X' is the group --C(R.sup.1).sub.2 --, with R.sup.1 being
hydrogen or (C.sub.1 -C.sub.4) alkyl, or the grouping
##STR17##
5. A membrane as claimed in claim 1, comprising
poly-N-vinylpyrrolidone and
at least one copolyaramide having at least three randomly recurring
structural units of the formula I, wherein
E.sub.1 is a divalent p-phenylene group,
E.sub.2 in the three recurrent structural units is one each of a divalent
p-phenylene group, a group of the formula
##STR18##
with R.sup.2 being --CH.sub.3, --OCH.sub.3, F, Cl or Br, and a group of
the formula
##STR19##
in which X' has the above-indicated meaning.
6. A membrane as claimed in claim 5, wherein the copolyaramide has the
recurrent structural units
##STR20##
7. A membrane as claimed in claim 1, wherein the aromatic polyamide is a
random copolymer, a block copolymer or a graft copolymer.
8. A membrane as claimed in claim 1, wherein the homogeneously miscible
polymer blend is prepared by polycondensing aromatic diamines and aromatic
dicarboxylic acids or their derivatives which are capable of undergoing
polycondensation, in the presence of polyvinylpyrrolidone.
9. A membrane as claimed in claim 1, comprising about 1 to 80% by weight of
polyvinylpyrrolidone relative to the sum of components.
10. A membrane as claimed in claim 9, comprising 5 to 60% by weight of
polyvinylpyrrolidone, relative to the sum of components.
11. A membrane as claimed in claim 1, which is asymmetric.
12. A membrane as claimed in claim 1, wherein said homogeneously miscible
polymer blend consists essentially of the recited ingredients.
13. A membrane as claimed in claim 1, wherein the aromatic polyamide has a
Staudinger index of about 50 to 1000 cm.sup.3 /g.
14. A process for the production of a membrane as claimed in claim 1, which
comprises the steps of:
a) providing a solution comprising a solvent and said homogeneously
miscible polymer blend, wherein said solvent comprises an aprotic, polar
solvent of the amide type;
b) spreading said solution as a liquid layer on a planar substrate; and
c) applying to said liquid layer a precipitation liquid which is miscible
with the solvent of said solution but in which said dissolved
homogeneously miscible polymer blend is precipitated as a membrane.
15. A process as claimed in claim 14, wherein said solvent is
N,N-dimethylacetamide or N-methyl-2-pyrrolidone.
16. A process as claimed in claim 14, wherein part of said solvent is
evaporated prior to coagulation into a membrane.
17. A process as claimed in claim 14, wherein said precipitation liquid is
water.
18. A process as claimed in claim 14, wherein said precipitation liquid is
allowed to act on the membrane precipitated thereby until virtually all
said solvent has been replaced therein by precipitation liquid.
19. A process as claimed in claim 18, wherein said membrane is freed from
said precipitation liquid by being dried in a stream of air.
20. A process as claimed in claim 19, wherein said membrane is treated,
before drying, with a plasticizer, and is then dried.
21. A process as claimed in claim 20, wherein said plasticizer is glycerol.
22. A process for modifying the retention capacity of a membrane produced
by a process as claimed in claim 20, comprising the step of subjecting
said membrane to heat treatment in warm air of about 20 to 100% relative
atmospheric humidity.
23. A process as claimed in claim 22, wherein said heat treatment is
carried out at a temperature of about 60.degree. to 220.degree. C. over a
period of about 0.1 to 96 hours.
24. A process as claimed in claim 19, wherein said membrane is dried at a
temperature of about 50.degree. C.
25. A process for modifying the retention capacity of a membrane produced
by a process as claimed in claim 18, comprising the step of subjecting
said membrane to heat treatment.
26. A process as claimed in claim 25, wherein said heat treatment is
carried out in a liquid.
27. A process as claimed in claim 26, wherein said liquid is an inert
liquid.
28. A process as claimed in claim 25, where heat treatment is carried out
with steam.
29. A process as claimed in claim 25, whereon said heat treatment is
carried out at a temperature of about 60.degree. to 220.degree. C. over a
period of about 0.1 to 96 hours.
Description
BACKGROUND OF THE INVENTION
The invention relates to a semipermeable membrane made from a homogeneously
miscible polymer blend and to a process for its production.
Since the introduction of asymmetric membranes made from cellulose acetate
by Loeb and Sourirajan (S. Sourirajan, Reverse Osmosis, Logos Press,
London 1970) and made from hydrophobic polymers (U.S. Pat. No. 3,615,024),
numerous membranes, in particular for separations of low-molecular weight
and macromolecular components dissolved in water, have been developed and
proposed, and their structure and suitability have been described in the
literature (Desalination, 35 (1980), 5-20). They have also been
successfully tested in industry or for medical purposes.
Many of the membranes described have particularly advantageous properties
for achieving specific objectives. As a consequence of their chemical and
physical structure, each of the individual membranes can only be optimally
suitable for very specific separation problems. This gives rise to the
basic need for always developing new membranes for new problems.
EP-A-0 082 433 gives a clear description of the advantages and
disadvantages of already known membranes. Thus, there are, for example,
hydrophilic, asymmetric membranes made from cellulose acetate which have
satisfactory antiadsorptive properties, but which leave much to be desired
with respect to their thermal and chemical resistance. Membranes made from
polysulfones or similar polymers may have good thermal and chemical
resistance. There is, however, a pronounced tendency in membranes of this
type, due to the hydrophobic properties of the polymers employed, to
adsorb dissolved substances, causing the membrane to become more or less
blocked. Although the mixtures of polysulfone and polyvinylpyrrolidone
described in EP-A-0 082 433 eliminate the disadvantage caused by the
hydrophobic character of polysulfone, these mixtures are, however,
sensitive to the influence of organic solvents. There are also problems
when these membranes are used for the treatment of waste water, because
so-called silicone defoamers which may be present in the waste water will
block the membranes.
U.S. Pat. No. 4,051,300 describes mixtures of aromatic polyamides with
polyvinylpyrrolidone. However, the polyamides are said to have a limited
compatibility with the polyvinyl pyrrolidone. These membranes still need
improving with respect to their hydrophilic character.
Hydrophilic character and simultaneous resistance to solvents are found in
membranes of regenerated cellulose; however, these can be hydrolyzed
relatively easily in acidic or alkaline media, and, in addition, they are
easily attacked by microorganisms.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
semipermeable membrane which has pronounced hydrophilic properties, i.e.
is capable of absorbing considerable amounts of water, relative to its
total weight.
Another object of the present invention is to provide a semipermeable
membrane which is stable to hydrolyzing agents and oxidants, is thermally
stable, displays improved resistance to organic solvents in comparison to
membranes made from a hydrophobic polymer, exhibits low adsorption of
proteins, has good wettability, and is insensitive to the action of
microorganisms.
A further object of the present invention is to provide a process for
producing the foregoing membrane.
Yet another object of the present invention is to provide a process for
modifying the retention capacity of the foregoing membrane.
In accomplishing the foregoing objectives, there has been provided, in
accordance with one aspect of the present invention, a semipermeable
membrane comprising a homogeneously miscible polymer blend which comprises
an aromatic polyamide and polyvinyl pyrrolidone.
The aromatic polyamide is in particular formed of the following general,
recurrent structural units of the formula I:
##STR3##
wherein E.sup.1 and E.sup.2 are identical or different and are selected
from the groupings
##STR4##
--Ar.sup.1 --, and --Ar.sup.1 --X--Ar.sup.2 ,
where Ar.sup.1 and Ar.sup.2 are the same or different 1,2-phenylene,
1,3-phenylene or 1,4-phenylene groups which may be substituted by (C.sub.1
-C.sub.6)-alkyl, (C.sub.1 -C.sub.6)-alkoxy, --CF.sub.3 or halogen and X
denotes
a) a direct bond or one of the following divalent groups: --O--,
--C(CF.sub.3).sub.2 --, --SO.sub.2 --, --CO--, --C(R.sup.1).sub.2 --, with
R.sup.1 being hydrogen, (C.sub.1 -C.sub.6)-alkyl or fluoroalkyl having
from 1 to 4 carbon atoms in the alkyl group or
b) --Z--Ar.sup.1 --Z--, where Z is one of the groups --O-- and
--C(CH.sub.3).sub.2 -- or
c) --O--Ar.sup.1 --Y--Ar.sup.2 --O--, where Y has the meaning given under
Xa) above.
In accordance with another aspect of the present invention there is
provided a process for the production of a membrane as described above,
which comprises the steps of: providing a solution comprising a solvent
and the foregoing homogeneously miscible polymer blend, wherein said
solvent comprises an aprotic, polar solvent of the amide type; spreading
the solution as a liquid layer on a planar substrate; and applying to the
liquid layer a precipitation liquid which is miscible with the solvent of
the solution but in which the dissolved homogeneously miscible polymer
blend is precipitated as a membrane.
In accordance with yet another aspect of the present invention, there is
provided a process for modifying the retention capacity of a membrane
formed by the foregoing process, wherein the membrane, in which virtually
all the solvent has been replaced by precipitation liquid, is subjected to
heat treatment. Preferably, the heat treatment is carried out in a liquid
or with steam.
Other objects, features and advantages of the present invention will become
apparent to those skilled in the art from the following detailed
description. It should be understood, however, that the detailed
description and specific examples, while indicating preferred embodiments
of the present invention, are given by way of illustration and not
limitation. Many changes and modifications within the scope of the present
invention may be made without departing from the spirit thereof, and the
invention includes all such modifications.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The aromatic polyamide which is appropriately employed for the membrane
according to the invention may be in the form of a random copolymer and
also in the form of a block copolymer or a graft copolymer.
Compounds which are suitable for the preparation of the aromatic polyamides
comprising recurrent structural units of the formula I are, in particular,
as follows:
Suitable dicarboxylic acid derivatives of the formula
Cl--CO--Ar.sup.1 --CO--Cl
are, for example, 4,4'-diphenyl sulfone dicarbonyl dichloride,
4,4'-diphenyl ether dicarbonyl dichloride, 4,4'-diphenyldicarbonyl
dichloride, 2,6-naphthalenedicarbonyl dichloride, isophthaloyl dichloride,
but very particularly terephthaloyl dichloride and substituted
terephthaloyl dichloride, for example 2-chloroterephthaloyl dichloride.
Suitable aromatic diamines of the structure H.sub.2 N--Ar.sup.1 --NH.sub.2
comprise m-phenylenediamines or substituted phenylenediamines, for example
2-chlorophenylenediamine, 2,5-dichlorophenylenediamine or
2-methoxy-p-phenylenediamine, in particular p-phenylenediamine.
Suitable substituted benzidine derivatives include 3,3'-dimethoxybenzidine,
3,3'-dichlorobenzidine, 2,2'-dimethylbenzidine and, preferably,
3,3'-dimethylbenzidine.
Suitable diamine components of the formula
H.sub.2 N--Ar.sup.1 --X--Ar.sup.2 --NH.sub.2
are, for example: 4,4'-diaminobenzophenone, bis(4-aminophenyl)-sulfone,
bis[4-(4'-aminophenoxy)phenyl]-sulfone, 1,2-bis(4'-aminophenoxy)-benzene,
1,4-bis[(4'-aminophenyl)isopropyl]-benzene,
2,2'-bis[4-(4'-aminophenoxy)phenyl]-propane, in particular,
1,4-bis-(4'-aminophenoxy)-benzene and mixtures of the diamines mentioned.
Blends which are advantageously used for preparing preferred embodiments of
membranes according to the invention include blends wherein the grouping
E.sup.1 comprises identical or different structural units and denotes a
1,3- or 1,4-phenylene group or the group
##STR5##
Blends wherein the grouping E.sup.2 comprises identical or different
structural units and denotes the 1,4-phenylene group or the group
##STR6##
wherein R.sup.2 denotes a lower alkyl or alkoxy group having up to 4
carbon atoms each in the alkyl group or F, Cl or Br or the group
##STR7##
in which X' is the group --C(R.sup.1).sub.2 --, with R.sup.1 being
hydrogen or (C.sub.1 -C.sub.4) alkyl, or the grouping
##STR8##
are also preferred.
Blends comprising
a) poly-N-vinylpyrrolidone and
b) at least one copolyaramide having at least three randomly recurring
structural units of the formula I, wherein
E.sub.1 is a divalent p-phenylene group,
E.sub.2 in the three recurrent structural units is one each of a divalent
p-phenylene group, a group of the formula
##STR9##
with R.sup.2 being --CH.sub.3, OCH.sub.3, F, Cl or Br, and a group of the
formula
##STR10##
in which X' has the above-indicated meaning, are preferred, as are blends
wherein the copolyaramide has the recurrent structural units
##STR11##
Polyaramides can be prepared in a known manner by solution condensation,
interfacial condensation or melt condensation.
The solution condensation of the aromatic dicarboxylic acid dichlorides
with the aromatic diamines is carried out in aprotic, polar solvents of
the amide type, such as, for example, in N,N-dimethylacetamide or in
particular in N-methyl-2-pyrrolidone. If appropriate, halide salts from
the first and/or the second group of the periodic table can be added to
these solvents in a known manner in order to increase the dissolving power
or to stabilize the polyamide solutions. Preferred additives are calcium
chloride and/or lithium chloride.
The polycondensation temperatures are usually between about -20.degree. C.
and +120.degree. C., preferably between +10.degree. C. and +100.degree. C.
Particularly good results are achieved at reaction temperatures between
+10.degree. C. and +80.degree. C. The polycondensation reactions are
preferably carried out in a manner such that about 2 to 30% by weight,
preferably 6 to 15% by weight, of polycondensate are present in the
solution after completion of the reaction.
The polycondensation can be stopped in a customary manner, for example by
adding monofunctional compounds such as benzoyl chloride.
After completion of the polycondensation, i.e. when the polymer solution
has reached the Staudinger index necessary for further processing, the
hydrogen chloride which has been produced and is loosely bound to the
amide solvent is neutralized by addition of basic substances. Examples of
suitable substances for this purpose are lithium hydroxide and calcium
hydroxide, and in particular calcium oxide.
The Staudinger index is a measure of the mean chain length of the polymers
produced.
The Staudinger index of the membrane-forming aromatic polyamides should be
between about 50 and 1,000 cm.sup.3 /g, preferably between 100 and 500
cm.sup.3 /g, particularly preferably between 150 and 350 cm.sup.3 /g. It
was determined on solutions each containing 0.5 g of polymer in 100 ml of
96% strength sulfuric acid at 25.degree. C.
The Staudinger index [.eta.] (intrinsic viscosity) is taken to mean the
term
##EQU1##
where
##EQU2##
C.sub.2 =concentration of the dissolved substance .eta.=viscosity of the
solution
.eta..sub.1 =viscosity of the pure solvent.
The blends according to the present invention can be prepared in a
customary manner from a common solution of PVP and a polyaramide in an
aprotic organic solvent, for example, dimethylformamide,
dimethylsulfoxide, N-methylpyrrolidone or N,N-dimethylacetamide. The
following methods can, for example, be chosen:
1.
a) Polycondensation of a polyaramide by means of solution condensation,
interfacial condensation or melt condensation,
b) dissolving the resulting polyaramide,
c) dissolving PVP and
d) thereafter mixing the PVP solution with the polyaramide solution.
2.
a) Solution condensation of a polyaramide and
b) subsequently adding dry PVP or a solution of PVP directly to the
composition for polycondensation.
3. It has surprisingly been found that the solution condensation of a
polyaramide can take place in the presence of PVP and that homogeneous
mixtures can thus also be obtained. The diamines are dissolved together
with PVP and a PVP/polyaramide solution is condensed by the addition of
dicarboxylic acid dichlorides.
By removing the solvent, e.g. by evaporation, the blends can be isolated
and further processed into intermediate products (granules or powder)
which can then be used as raw materials for the production of membranes.
The molecular weight of the PVP, specified as the mean weight, is generally
about 1,000 to 3,000,000, preferably about 40,000 to 200,000, in
particular about 50,000 to 100,000.
The blends of the present invention are homogeneously miscible in any
mixing ratio. They contain, in particular, PVP in quantities ranging from
about 1 to 80% by weight, preferably from 5 to 60% by weight and
particularly preferably from 10 to 50% by weight, relative to the sum of
components (a+b).
The polymer blends described above are not as such the subject matter of
the present invention; rather, they are described in detail in connection
with moldings, in a patent application of the same priority date. Instead,
the invention relates to a semipermeable membrane containing the polymer
blend mentioned as the principal component.
In order to produce the membrane according to the invention from the
polymer blend, the above-described solution of the blend is filtered and
degassed, and a semipermeable membrane is then produced in a known manner
by phase inversion (Robert E. Kesting, "Synthetic Polymeric Membranes",
2nd Ed., 1985, p. 237 et seq.). To this end, the polymer solution is
spread as a liquid layer on a substrate which is as planar as possible.
The planar substrate can comprise, for example, a glass plate or a metal
drum.
A precipitation liquid which is miscible with the solvent of the solution,
but in which the polymers dissolved in the polymer solution are
precipitated as a membrane is then allowed to act on the liquid layer. An
example of a precipitation liquid used is water. Due to the action of the
precipitation liquid on the liquid layer comprising the polymer solution,
the substances dissolved therein precipitate to form a semipermeable
membrane.
When carrying out the process, the precipitation liquid is advantageously
allowed to act on the membrane precipitated thereby until virtually all
the solvent has been replaced by precipitation liquid. The membrane formed
is then freed from precipitation liquid, for example by drying the
membrane directly in a stream of air or alternatively by first treating
the membrane with a plasticizer, such as glycerol, and then drying it.
To produce membranes which are arranged on a support layer which is
permeable to flowable media, the above-mentioned procedures are followed,
but a non-woven, for example made of plastic, or a paper is used as the
substrate to form the membrane layer and serves as a support for the
latter, and the membrane layer formed is left on this substrate. However,
it is also possible first to produce the membrane without a support and
only then to apply it to a permeable support.
Hollow filaments or capillaries can also be produced in a known manner from
the solution of the polymer blend by spinning the polymer solution in
accordance with the prior art through an appropriately constructed shaping
annular die or hollow-needle nozzle into the precipitation liquid.
According to the prior art, the production conditions can here be chosen
in such a way that an external skin or an internal skin or both are
formed. The wall thickness of capillaries or hollow fibers of this type is
usually in the range from about 20 to 500 .mu.m.
If the membrane is impregnated with glycerol after coagulation, it can
preferably contain in the range from about 5 to 60% glycerol, based on its
total weight; the membrane impregnated in this way is dried, for example
at a temperature of 50.degree. C.
The membrane according to the invention is likewise suitable as a support
membrane for permselective layers produced directly on or in the membrane.
Thus, for example, "ultrathin" layers (<1 .mu.m) made from polymers
containing functional groups (for example silicones, cellulose ethers or
fluorinated copolymers) can be spread on water, applied therefrom onto the
membrane surface and bound covalently, for example by reaction with a
diisocyanate, in order to thus achieve higher permselectivities.
Analogously, the membrane according to the invention is also suitable as a
support for reactive molecules, for example in order to immobilize enzymes
or anticoagulants such as heparin in accordance with the prior art.
The thickness of the membrane according to the invention without a support
layer is in the range from about 10 to 300 .mu.m, in particular 20 to 120
.mu.m.
The invention is described in greater detail below with reference to
illustrative embodiments, but without the embodiments given therein
representing a limitation.
EXAMPLES 1 TO 7
For the production of the membranes investigated in the examples,
copolyaramide I was first prepared in N-methylpyrrolidone as solvent from
(A') about 95 to 100 mol-% of terephthaloyl dichloride (TPC),
(B') 25 mol-% of para-phenylenediamine (PPD),
(C') 50 mol-% of 3,3'-dimethylbenzidine (DMB) and
(D') 25 mol-% of 1,4-bis-(4-aminophenoxy)benzene (BAPOB) at a temperature
of 50.degree. C.
In the same way, polyaramide II was prepared from about 95 to 100 mol-% of
terephthaloyl dichloride and 100 mol-% of
bis[4-(4-aminophenoxy)-phenyl]sulfone.
After neutralizing with 100 mol-% of CaO various quantities of
poly-N-vinylpyrrolidone in a solid state were added with stirring to these
solutions. The resulting clear solutions having various Staudinger indices
and with various concentrations (for more precise data see Table 1) were
then applied to a polypropylene support non-woven (obtainable from Messrs.
Freudenberg: FO 2430.RTM. 100 g/m.sup.2) using a casting device in
accordance with U.S. Pat. No. 4,229,291, and coagulated in water at
14.degree. C. The membranes were then impregnated with an aqueous solution
of 40% by weight of glycerol and dried at 50.degree. C. The dry
support-reinforced membranes had a thickness of 280 .mu.m.
Surprisingly, the membrane properties can subsequently be modified by
heat-treating the membrane. In Examples 2 and 4, it is shown how it is
possible to substantially increase the retention capacity for dissolved
substances by placing the membrane in hot water (100.degree. C.).
The membrane properties of the membranes produced in this way are given in
Table 1 below.
The Staudinger index for the aromatic polyaramide was determined in 96%
strength H.sub.2 SO.sub.4 at 25.degree. C. as specified in the
description.
The mechanical permeability (ultrafiltration) and the retention capacity
for dissolved macromolecules were determined in a stirred cylindrical cell
(700 rpm, 350 ml, membrane surface area 43 cm.sup.2) at pressures of 3.0
bar at 20.degree. C. The retention capacity is defined as
##EQU3##
C.sub.1 is the concentration of the aqueous test solution, C.sub.2 is the
concentration in the permeate.
The test solution employed was a 2% strength aqueous polyvinylpyrrolidone
solution (PVP), obtainable under the name "Kollidon K30".RTM. from Messrs.
BASF, and the molecular weight of the polyvinylpyrrolidone was 49,000
Daltons.
The concentrations were measured in a digital densitometer "DMA
60+601".RTM. from Messrs. Heraeus.
TABLE 1
__________________________________________________________________________
Ex- Staudinger Index
Polymer Concentration
Concentration
Mechanical
Retention
am- of Polyaramide
Concentration
of PVP of CaCl.sub.2
Permeability
Capacity
Test
ple
Polyaramide
(ml/g) (%) (%) (%) (l/m.sup.2 .multidot.
(%) Substance
__________________________________________________________________________
1 Copolyaramide I
188 6.0 4.0 (K30)
2.8 460 87 PVP K30
.sup. 2.sup.a)
Copolyaramide I
188 6.0 5.2 (K30)
2.8 170 95 PVP K30
3 Copolyaramide I
220 7.0 3.1 (K30)
3.2 105 71 Dex T10.sup.c)
.sup. 4.sup.b)
Copolyaramide I
220 7.0 6.0 (K30)
3.2 75 97 Dex T10.sup.c)
5 Copolyaramide I
250 6.0 3.9 (K30)
3.0 265 89 PVP K30
6 Copolyaramide I
188 5.0 5.2 (K90)
2.4 360 78 PVP K30
7 Polyaramide II
155 8.0 5.8 (K30)
1.6 680 73 PVP
__________________________________________________________________________
K30
All concentrations given in percent by weight/total weight of solution
.sup.a) thermal posttreatment: 1 h in water at 100.degree. C.
.sup.b) thermal posttreatment: 9 h in water at 100.degree. C.
.sup.c) Dextran 10,000 (Pharmacia)
EXAMPLES 8 TO 10
In order to test the solvent resistance of the membranes according to the
invention, the membranes of Examples 1 to 3 were placed in acetone for 1
hour in order to replace the liquid present in the membrane pores by
acetone. The membranes were then exposed to the solvents given in Table 2
for a period of 12 hours at a temperature of 25.degree. C. The membranes
were then reconditioned to water, and the mechanical permeability and the
retention capacity of the membranes treated with the organic solvents were
then measured as stated under Example 1. The results are given in Table 2
and show that the differences from the values given in Table 1 are within
the tolerance limits of the measurement method.
TABLE 2
______________________________________
Ex- Membrane Mech. Per-
Retention
am- from Sol- meability
Capacity
Test
ple Example vent (l/m.sup.2 .multidot. h)
(%) Substance
______________________________________
8 1 toluene 340 88 PVP K30
9 2 CHCl.sub.3
150 95.5 PVP K30
10 3 ethyl 95 69 DEXTRAN
acetate T10
______________________________________
EXAMPLES 11 TO 17
Aqueous solutions (0.05%) of the colored protein cytochrome C in a stirred
cell were subjected to ultrafiltration using the membranes of Examples 1
to 7. After a test period of 30 minutes, the membranes were thoroughly
washed with a buffer solution (pH 6.8). The membranes did not show any
staining with red cytochrome C, which indicated a low adsorption of
proteins.
Membranes having the same molecular weight cut-off, but which were made
from various aromatic polyamides or polysulfone, on the other hand,
exhibited a strong adsorption of proteins.
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